1.
A PFRCSTR combination is used to carry out a first order reaction.
The PFR is
placed in series before a CSTR.
The feed concentration to the PFR is
C
A
0
= 4 mol m
3
and the conversion in the PFR is 65 %.
The rate constant is 0.08 s
1
and the feed rate
of A is 3 mol s
1
.
If the CSTR volume is 1 m
3
, what is the concentration of A after the
CSTR?
Hence, what is the overall conversion for the PFRCSTR combination?
Solution
: First draw a diagram:
First we calculate concentration and molar flowrate after the PFR
C
A1
= C
A0
(1X
A1
) = 4(10.65) = 1.4 mol m
3
F
A1
= F
A0
(1X
A1
) = 3(10.65) = 1.05 mol s
1
Next we must apply the CSTR design equation to the stirred tank.
But we must note that the
feed to the stirred tank is the outlet stream from the plug flow reactor, and therefore the design
equation is:
)
(
2
1
2
A
A
A
R
r
X
F
V

′
=
Where in these notes,
2
A
X
′
denotes local conversion change across the CSTR.
X
A2
is reserved
to represent overall conversion in the PFRCSTR combination.
As with all applications of the design equation, we have to substitute in a rate equation, in this
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 Spring '10
 reddy
 Reaction, Trigraph, Rate equation, Cubic foot, 0.035 M

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